1. Field of the Invention
The present invention relates to field effect transistors and, more particularly, to circuits for driving field effect transistors.
2. Description of the Related Art
High power field effect transistors (FETs) may be driven by what is referred to as Pre-FET drivers in the form of integrated circuits (ICs). Such Pre-FET drivers may also function to protect the FETs in case of fault conditions such as short-to-battery and short-to-ground, etc. For example, FETs are often used in automotive electronic devices, such as engine control modules (ECMs). If a load connected to the FET shorts out, or if a service technician inadvertently connects a wiring harness in a wrong orientation, it is possible that a FET may be directly,connected to battery voltage, which may result in costly damage to the FET. Another possible function of the Pre-FET driver is to report the fault status back to the microprocessor for further processing.
A problem is that Pre-FET driver ICs are relatively costly. Depending upon the number of channels used, i.e., the number of FETs to be driven, the price of a Pre-FET driver can vary from $0.70 to $1.40. Pre-FET drivers are inflexible in that they are configured to drive four, six or eight channels. Thus, even if only one FET needs to be driven, the minimum configuration of a four channel driving device must be used, which is a relatively costly way to drive a single FET.
Another problem with Pre-FET driver ICs is that each of the channels has a common short-to-battery detection voltage threshold. Thus, it is difficult to interface one single pre-FET driver with a group of FETs that have very large differences in their internal resistances.
Yet another problem with Pre-FET driver ICs is that they require a fixed turn-off time to switch off their outputs in order to protect the FETs in the event of a short-to-battery. More particularly, during and after turn-on, the drain voltages of the external FETs are compared against a fixed short-to-battery threshold voltage, which is generated outside the IC. If the drain voltage exceeds the short-to-battery threshold voltage, then the gate drive will be disabled after a fixed duration. This short-to-battery detection and disable time can vary from 60 to 90 μsec. During this time, the peak short circuit current can rise up to a maximum of 28 to 40 Amps, which can result in damage to the FETs.
What is needed in the art is an inexpensive method of driving power FETs which overcomes the problems associated with Pre-FET driver ICs.